gremlins

Gremlins are imaginary mischievous creatures blamed for unexplained mechanical failures or technical glitches.

The term emerged among British pilots in World War II to explain mysterious aircraft malfunctions: There must be gremlins in the system.

Now applied broadly to any frustrating technical problem with no obvious cause: Software gremlins kept crashing the app. (Capitalized Gremlins is the 1984 film.)